If a battery will deliver sufficient amps from a 3s battery to drive a motor at "X" RPM, ...
Not to detract from Igor's response, which is entirely correct in a nicely practical way and should be heeded in every regard, I have a physics/electrical nit I must pick:
Batteries are, roughly, constant voltage sources. The voltage they can supply drops with increasing current, but a LiPo cell's voltage stays within +/-20% of it's rated voltage throughout a flight. Motors, to a similar degree of roughness, put out a given RPM for an applied voltage (or if they aren't, they're so overloaded they quickly burn up). So you really want to think about whether the battery can deliver sufficient
voltage to drive a motor at "X" RPM.
But you need to think about whether the battery can do this when it is at the end of a flight, and at those portions of the flight envelope where the most current is being demanded (this happens even with a constant-RPM setup, when the plane gets significantly slower than cruise speed). Battery voltage sags with current, it sags more with current when it's close to discharge, and it has less voltage to start with when it's close to discharged. This is why Igor suggests his ground test with a battery close to the state of charge it'll be at the end of a flight -- running the plane on the ground asks for a bit (but not a bunch) more current than the worst-case load in the air, so if the thing will hold RPM with a battery at end-of-flight charge when it's on the ground, you're fine.
I'm not sure how this works with the Igor timer. I
think it has the RPM command at maximum on the ground, because it knows the plane isn't moving, but I wouldn't guarantee it -- particularly because Igor is right here to clarify things.